epidemic ep·i·dem·ic (ěp'ĭ-děm'ĭk) or ep·i·dem·i·cal (-ĭ-kəl)adj. Spreading rapidly and extensively by infection and affecting many individuals in an area or a population at the same time, as of a disease or illness. n. An outbreak or unusually high occurrence of a disease or illness in a population or area.

an occurrence of disease that is temporarily of high prevalence. An epidemic occurring over a wide geographical area (e.g., worldwide) is called a pandemic. The rise and decline in epidemic prevalence of an infectious disease is a probability phenomenon dependent upon transfer of an effective dose of the infectious agent from an infected individual to a susceptible one. After an epidemic has subsided, the affected host population contains a sufficiently small proportion of susceptible individuals that reintroduction of the infection will not result in a new epidemic. Since the parasite population cannot reproduce itself in such a host population, the host population as a whole is immune to the epidemic disease, a phenomenon termed herd immunity.